Electronic circuits and systems often require more than one regulated operating voltage. One way to provided multiple regulated voltages is to provide a plurality of regulating power converters, each regulating power converter providing one of the required operating voltages. Another way is to provide a multi-output power converter that is arranged to provide more than one regulated voltage output.
One example of a multi-output power converter is a forward switching power converter comprising a transformer with a plurality of secondary windings: because of the characteristic of forward converters, each secondary winding may require a dedicated regulator (e.g., linear, switching, magnetic) to maintain its respective voltage output in regulation.
Another example of a multi-output power converter is shown in FIG. 1. In FIG. 1, a flyback converter 300 comprises a transformer 302 having a single primary winding 303 and multiple secondary windings 304a, 304b. When switch 305 is turned on by switch controller 310, the input voltage, Vin, is impressed across primary winding 303; rectifiers 306a, 306b are reverse biased and non-conductive; and magnetic energy is stored in transformer 302. When switch 305 is turned off by switch controller 310, the voltages across the windings reverse, the rectifiers conduct and the stored energy is transferred to storage capacitors 307a, 307b and loads 308a, 308b. The switch controller 310 varies the duty cycle of switch 305 as a means of maintaining a first output voltage, across load 308a, at an essentially fixed value Vo. Storage capacitor C1 307a smoothes the output voltage Vo.
During the time that the switch 305 is off, and assuming ideal components (e.g., no voltage drops across diodes 306a, 306b; no winding resistances or leakage inductance in the transformer 302) the voltage across secondary winding 304a will be clamped to Vo and the voltage across secondary winding 304b will be V2=(N3/N2)*Vo, where N3 and N2 are the number of turns on windings 304a and 304b, respectively. Assuming ideal components and a sufficiently large storage capacitor C2 307b, the second output voltage, across load 308b, will also be essentially equal to V2.
The example of FIG. 1 may be extended to comprise several secondary windings, each winding having a relative number of turns that is selected to provide a particular value of output voltage. By this means, regulation of a single output may be used to generate a plurality of other regulated outputs of different values.
In practice, cross-regulation (as used herein, the term “cross regulation” in a multiple-output power converter shall refer to the dependency of one or more voltage outputs on the values of one or more other voltage outputs) in a multiple-output flyback regulator is affected by the non-ideal nature of components. Transformer resistances and leakage inductances, and voltage drops across rectifiers, typically cause degradation in the cross-regulation performance of the converter.
Cross-regulation may also depend upon the relative loading on different outputs. In the converter of FIG. 1, for example, the total energy available for delivery to the first output (at voltage Vo) and the second output (at voltage V2) during each converter operating cycle is the amount of energy stored in transformer 303 during the on-time of switch 305. The duty cycle of the converter (e.g., the fraction of each converter operating cycle that switch 305 is turned on), however, decreases as the power delivered to the first output (i.e., to load 308a) decreases. If the first output is relatively lightly loaded, or unloaded, and the second output is relatively heavily loaded, there may not be sufficient energy stored in the transformer during each converter operating cycle to maintain the second output at the voltage V2.
A variety of methods have been proposed to improve cross-regulation in multiple-output power supplies. Examples of such methods may be found in Marrero, Improving Cross Regulation of Multiple Output Flyback Converters, PCIM Conference Proceedings 1996; Schwartz, Synchronously Rectified Buck-Flyback DC to DC Power Converter, U.S. Pat. No. 5,552,695; and Gan et al, Multiple Output Converter with Improved Cross Regulation U.S. Pat. No. 6,987,679.